Ellipsometer focusing system

Abstract

An ellipsometer includes an integrated focusing system with a beam splitter between the sample and the ellipsometer detector. The beam splitter provides a portion of the radiation to a lens system that magnifies any deviation from a best focus position by at least 2×. The focusing system includes a 2D sensor, where the spot of light focused on the sensor is 50 percent or smaller than the sensor. The focusing system may further include a compensator to correct optical aberrations caused by the beam splitter. A processor receives an image signal and finds the location of the spot from which focus error can be determined and used to correct the focal position of the ellipsometer. The processor compensates for movement of the spot caused by rotating optics. Additionally, a proportional-integral-derivative controller may be used to control exposure time and/or gain of the camera.

What is claimed is:  
     
       1. A method of focusing an ellipsometer, the method comprising:
 generating radiation; 
 polarizing the radiation to produce a sample beam that is incident on and reflected by a sample to produce reflected radiation; and 
 focusing the sample beam on the sample comprising:
 focusing a portion of the reflected radiation into a spot on a two-dimensional sensor, the portion of the reflected radiation including an entire cross-section of the reflected radiation; 
 determining a location of the spot on the two-dimensional sensor; 
 determining a deviation from a best focus position using the location of the spot on the two-dimensional sensor; and 
 adjusting a focal position of the ellipsometer based on the deviation from the best focus position. 
 
 
     
     
       2. The method of  claim 1 , wherein focusing the portion of the reflected radiation into a spot on the two-dimensional sensor provides a magnified indication of at least 2× of any deviation from the best focus position of the ellipsometer and produces the spot that is 50 percent or smaller than the two-dimensional sensor. 
     
     
       3. The method of  claim 1 , further comprising splitting the portion of the reflected radiation from the reflected radiation and compensating for optical aberrations in the reflected radiation caused by splitting the portion of the reflected radiation. 
     
     
       4. The method of  claim 1 , wherein determining a location of the spot on the two-dimensional sensor comprises finding a centroid of the spot found. 
     
     
       5. The method of  claim 1 , wherein the location of the spot with respect to a center of the two-dimensional sensor is mis-aligned with the best focus position of the ellipsometer, the method further comprising compensating for mis-alignment of the location of the spot from the center of the two-dimensional sensor. 
     
     
       6. The method of  claim 1 , further comprising rotating polarization of the radiation by rotating an optical element that causes the location of the spot on the two-dimensional sensor to move as the optical element rotates, the method further comprising compensating for movement of the location of the spot on the two-dimensional sensor caused by rotation of the optical element. 
     
     
       7. The method of  claim 1 , further comprising controlling at least one of exposure time and gain for the two dimensional sensor using a proportional-integral-derivative controller. 
     
     
       8. A method of focusing an ellipsometer, the method comprising:
 producing a polarized sample beam that is incident on a sample, wherein reflected radiation is reflected by the sample; 
 splitting a portion of the reflected radiation from the reflected radiation, the portion of the reflected radiation including an entire cross-section of the reflected radiation; 
 focusing a portion of the reflected radiation into a spot on a two-dimensional sensor; 
 determining a location of the spot on the two-dimensional sensor; 
 determining a deviation from a best focus position using the location of the spot on the two-dimensional sensor; and 
 adjusting a focal position of the ellipsometer based on the deviation from the best focus position. 
 
     
     
       9. The method of  claim 8 , wherein focusing the portion of the reflected radiation into a spot on the two-dimensional sensor provides a magnified indication of at least 2× of any deviation from the best focus position of the ellipsometer and produces the spot that is 50 percent or smaller than the two-dimensional sensor. 
     
     
       10. The method of  claim 8 , further comprising compensating for optical aberrations in the reflected radiation caused by splitting the portion of the reflected radiation. 
     
     
       11. The method of  claim 8 , wherein determining a location of the spot on the two-dimensional sensor comprises finding a centroid of the spot found. 
     
     
       12. The method of  claim 8 , further comprising rotating polarization of the radiation by rotating an optical element that causes the location of the spot on the two-dimensional sensor to move as the optical element rotates, the method further comprising compensating for movement of the location of the spot on the two-dimensional sensor caused by rotation of the optical element. 
     
     
       13. The method of  claim 8 , further comprising controlling at least one of exposure time and gain for the two dimensional sensor using a proportional-integral-derivative controller. 
     
     
       14. An ellipsometer comprising:
 a source that emits radiation; 
 a polarization state generator for polarizing the radiation to produce a sample beam that is incident on and reflected by a sample; 
 a polarization state analyzer positioned to receive reflected radiation from the sample; 
 a detector positioned to receive the reflected radiation after the reflected radiation passes through the polarization state analyzer; and 
 a focusing system to focus the sample beam on the sample, the focusing system comprising:
 a beam splitter in a beam path between the sample and the detector, the beam splitter positioned to receive the reflected radiation and to provide a portion of the reflected radiation that includes an entire cross-section of the reflected radiation; 
 a lens system that receives the portion of the reflected radiation, the lens system provides a magnified indication of any deviation from a best focus position of the ellipsometer; 
 a two-dimensional sensor positioned to receive the portion of the reflected radiation from the lens system as a spot, the two-dimensional sensor generates an image signal of the spot; 
 
 a processor that receives the image signal and is configured to find a location of the spot on the two-dimensional sensor to determine the deviation from the best focus position of the ellipsometer using the location of the spot on the two dimensional sensor. 
 
     
     
       15. The ellipsometer of  claim 14 , wherein the two-dimensional sensor and the lens system in the focusing system produce a spot on the two-dimensional sensor that is 50 percent or smaller than the two-dimensional sensor. 
     
     
       16. The ellipsometer of  claim 14 , further comprising a compensator positioned in the beam path between the beam splitter and the detector, the compensator correcting optical aberrations in the reflected radiation caused by the beam splitter. 
     
     
       17. The ellipsometer of  claim 14 , further comprising stage servo controller coupled to the processor and that alters a focal position of the ellipsometer, wherein the stage servo controller alters the focal position of the ellipsometer based on the deviation from the best focus position. 
     
     
       18. The ellipsometer of  claim 14 , wherein the processor finds the location of the spot based on a centroid of the spot. 
     
     
       19. The ellipsometer of  claim 14 , wherein the ellipsometer comprises a rotating optic that causes the location of the spot to move on the two-dimensional sensor as the rotating optic rotates, wherein the processor is configured to compensate for movement of the location of the spot on the two-dimensional sensor caused by the rotating optic. 
     
     
       20. The ellipsometer of  claim 14 , further comprising a proportional-integral-derivative controller coupled to the two-dimensional sensor, the proportional-integral-derivative controller controlling at least one of exposure time and gain for the two-dimensional sensor.